This invention relates to an aluminum electrolytic capacitor containing an electrolyte having as solute a mono salt of azelaic acid and diisopropylamine or triethylamine. This electrolyte permits capacitor operation at 105.degree. C. and 150 VDC or higher.
Electrolytes for aluminum electrolytic capacitors operating at voltages of 150 V or higher most commonly contain salts of boric acid or boric acid derivatives in ethylene glycol. The maximum operating temperature for such an electrolyte system is less than 100.degree. C. and normally 65.degree.-85.degree. C. This temperature limitation is caused by the rapid reaction of glycol with boric acid and other borate species to form polymeric glycolborates and water at about 100.degree. C. The minimum operating temperature in such a system is above -20.degree. C. as glycol freezes at -17.4.degree. C.
It is desirable to develop electrolytic capacitors capable of operating continuously at 150 VDC or above at a temperature of 105.degree. to 125.degree.C. Electrolyte systems for such capacitors, described by M. Finkelstein, S. D. Ross, and F. S. Dunkl in U.S. Pat. Nos. 4,373,176, and 4,373,177, both issued Feb. 8, 1983, have been developed which utilize nonborate electrolytes because, as noted above, borates react with glycol.
It is desirable that that solute be stable at 105.degree. C. because of the internal temperature developed during operation of capacitors at 130 VDC and above. Screening tests in sealed tubes at 105.degree. C. and 125.degree. C. have proved satisfactory in predicting suitable capacitor operation, particularly in checking for resistivity increase which must be less than 25%.
The major cause of resistivity increase is amide formation, particularly when the solute is an ammonium or substituted ammonium salt of a monobasic or dibasic carboxylic acid. Amide formation is easiest with ammonium salts and decreases with increasing substitution in the amine, with tertiary amines least susceptible as then amide formation requires cleavage of the carbonnitrogen bond. Another possible degradative reaction is cyclic ketone formation, particularly C.sub.5 to C.sub.7 ketones, but is of little consequence for acids containing more than eight carbon atoms.
The isoelectric point of aluminum oxide, i.e. the point of maximum stability and minimum solubility, is at pH of 5.5. Thus, a slightly acidic solute as provided by a mono salt of a dibasic acid is less likely to attack the aluminum oxide dielectric than is the slightly basic di-salt.